These guys have a theoretical mileage estimator based on vehicle weight, aerodynamics, horsepower loss of trans, etc.
MPG Estimator
Based on cars more in my 'fleet' I made up some charts I thought I would post as some generalizations (some assumptions). You can see how horsepower loss to transmission, aerodynamics, and weight can effect where you will theoretically get the best gas mileage.
I didn't bother changing BSFC between the engines...
There are so many factors that could alter results, so take it with a grain of salt. But, perhaps it could be a good rough guide for theoretical speeds for gas mileage on a given platform. It also seems to show how slowing it down could make a big difference on a small car, or even a mid-large SUV.
